Likely lad sat on by old theologian (4-2)
I believe the answer is:
odds-on
'likely' is the definition.
(I've seen this before)
'lad sat on by old theologian' is the wordplay.
'lad' becomes 'son' (lad can informally mean a young boy).
'sat on by' means one lot of letters go next to another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'theologian' becomes 'DD' (Doctor of Divinity degree).
'o'+'dd'='odd'
'son' after 'odd' is 'ODDS-ON'.
(Other definitions for odds-on that I've seen before include "With success more likely than favour in bookie's phrase" , "Very likely to happen" , "Rated likely to win" , "With success more likely than failure - check with bookie" , "Bookie's assessment of success being more likely than failure" .)